1. Field of the Invention
This invention relates to a radiation image storage panel comprising a support, a phosphor layer which contains a stimulable phosphor and a protective film, superposed in this order.
2. Description of Prior Arts
For obtaining a radiation image, there has been conventionally employed a radiography utilizing a combination of a radiographic film having an emulsion layer containing a photosensitive silver salt and a radiographic intensifying screen.
As a method replacing the conventional radiography, a radiation image recording and reproducing method utilizing a stimulable phosphor as described, for instance, in U.S. Pat. No. 4,239,968, has been recently paid much attention. In the radiation image recording and reproducing method, a radiation image storage panel comprising a stimulable phosphor (i.e., stimulable phosphor sheet) is used, and the method involves steps of causing the stimulable phosphor of the panel to absorb radiation energy having passed through an object or having radiated from an object; sequentially exciting the stimulable phosphor with an electromagnetic wave such as visible light or infrared rays (hereinafter referred to as "stimulating rays") to release the radiation energy stored in the phosphor as light emission (stimulated emission); photoelectrically detecting the emitted light to obtain electric signals; and reproducing the radiation image of the object as a visible light from the electric signals.
In the radiation image recording and reproducing method, a radiation image is obtainable with a sufficient amount of information by applying a radiation to the object at considerably smaller dose, as compared with the conventional radiography. Accordingly, this method is of great value especially when the method is used for medical diagnosis.
The radiation image storage panel employed in the radiation image recording and reproducing method has a basic structure comprising a support and a phosphor layer provided on one surface of the support. Further, a transparent film is generally provided on the free surface (surface not facing the support) of the phosphorlayer to keep the phosphor layer from chemical deterioration or physical shock.
The phosphor layer comprises a binder and stimulable phosphor particles dispersed therein. The stimulable phosphor emits light (gives stimulated emission) when excited with an electromagnetic wave (stimulating rays) such as visible light or infrared rays after having been exposed to a radiation such as X-rays. Accordingly, the radiation having passed through or having radiated from an object is absorbed by the phosphor layer of the panel in proportion to the applied radiation dose, and a radiation image of the object is produced in the panel in the form of a radiation energy-stored image. The radiation energy-stored image can be released as stimulated emission by sequentially irradiating (scanning) the panel with stimulating rays. The stimulated emission is then photoelectrically detected to give electric signals, so as to reproduce a visible image from the electric signals.
When the radiation image recording and reproducing method is practically carried out, the radiation image storage panel is repeatedly used in a cyclic procedure comprising steps of exposing the panel to a radiation (i.e., recording a radiation image), irradiating the panel with stimulating rays (i.e., reading out the recorded radiation image), and exposing the panel to light for erasure (i.e., erasing the remaining energy from the panel). In the cyclic procedure, moving the panel from one step to the next step is done by a transfer system, and after one cycle is finished, the panel is usually piled upon other panels and stored.
More in detail, the radiation image storage panel is moved from one step to the next step through the transfer system which comprises rolls, transfer belts and guide plates combined in various manners in a radiation image recording and reproducing apparatus. In such transfer system, the panel is not always moved only in the longitudinal direction of the panel, and depending on the apparatus, the panel is moved in the lateral direction thereof or in both the directions. Accordingly, the protective film of the panel preferably has a large thickness from the viewpoint of mechanical strength such as bending strength, though the mechanical strength required for the protective film varies depending on the purpose of use of the panel, kind of the apparatus, etc.
On the other hand, the radiation image is read out from the transparent protective film-side surface of the radiation image storage panel by scanning the panel with stimulating rays, so that the thickness of the protective film is desired to be as small as possible from the viewpoint of image quality such as sharpness of the image. As the protective film of the panel having a small thickness and a high mechanical strength, there has been so far employed a polyethylene terephthalate film improved in the mechanical strength against bend or the like by means of stretching. However, the conventional polyethylene terephthalate film has a large difference between the strength in one direction and the strength in the direction perpendicular thereto (e.g., longitudinal direction and lateral direction), and the lateral strength has been liable to be small even when the longitudinal strength being satisfactory.
Particularly in the case that the radiation image storage panel is transferred in both of the longitudinal and lateral directions, the protective film tends to suffer damages such as cracks or wrinkles caused by bending or the like by means of rolls during the transfer in the lateral direction. Such damages give rise not only to decrease the function of the protective film per se but also to deteriorate the quality of resulting image. Accordingly, the panel is desired to be prevented from suffering damages on the protective film during the transfer.